Pipelines develop flaws over time. If left uncorrected, such flaws may eventually result in catastrophic failure of the pipeline. Such a catastrophic failure may result in lost services and revenues. Because a pipeline may fail without warning, early detection of flaws is fundamental to preventing catastrophic failure.
One method of inspection that has proven successful for pipelines in the field is the eddy-current technique. In the eddy-current technique, an electromagnetic field is induced within the pipeline. Flaws in the pipeline distort a component of this field. Analysis of these distortions locates and defines flaws in the pipeline.
In order to perform an in-field inspection, an electronic inspection system is passed through the pipeline under controlled conditions. The mechanics of passing an inspection system present several problems.
A problem exists in that many inspection systems contain components that are unable to negotiate sharp bends or junctions. These systems are therefore unsuitable for use with convoluted pipelines.
In addition, an inspection system that is unable to negotiate the bends and junctions in a pipeline is likely to become jammed in the pipeline. If a system becomes stuck within a pipeline, then the system itself becomes a “flaw” (i.e., a blockage) of the pipeline, necessitating repair.
Many inspection systems are configured to move in one direction only. Since any system may become stuck in the pipeline under a specific set of circumstances, there should be some way of backing the system out of the pipeline. Systems configured to move in only one direction are therefore undesirable.
Many inspection systems are constructed using materials that do support the growth of bacteria and/or fungi. Such systems may therefore be carriers of disease and parasites, and are therefore unsuitable where sanitary conditions must be maintained, as in a municipal water system or a food-processing facility.
Similarly, many inspection systems contain materials that pose a risk of contamination. For example, lubricants or materials that corrode or shed are inherently unsuitable for pipelines used in municipal water systems, or food- or chemical-processing facilities.
Conversely, many inspection systems contain materials that may be adversely affected by the normal-contents of the pipeline, i.e., the normal contents of the pipeline may corrode or degrade the materials of the system. A system with steel components, for example, would be entirely unsuitable for a pipeline that normally carries sulfuric acid.
Also, many inspection systems contain components, such as pull lines or housings, that may potentially damage the pipeline. For example, steel housings may scratch the inside of the pipeline, thereby producing potential future flaws.
An inspection system is limited in the length of pipeline inspected in one pass by its ability to move through the pipeline. A prime consideration in this area is friction. The easier a system can slip though the pipeline, the less friction it will generate. Heavy systems generate more friction than similar lightweight systems.
The negotiation of bends and junctions generates more friction than the negotiation of straight sections of pipeline. Cumbersome systems containing large components negotiate bends and junctions less readily than more streamlined systems with smaller components. Such cumbersome systems are therefore undesirable.
The material of which a system is made may have a severe effect upon the generated friction. Systems made of materials that exhibit a high frictional constant are therefore undesirable.
For inspection systems that are pulled through a pipeline by a towline, the towline may produce a significant amount of friction in and of itself. For example, it takes considerable force to simply drag a half-inch steel cable through a two-kilometer steel pipeline. In addition, the cable poses a significant hazard to the pipeline, especially at bends and junctions where the dragging of the cable may actually cut into the inner surface of the pipeline.
Similarly, an umbilical line is often used to power the electronic components of a system and bring out the resultant data. The umbilical line itself may generate significant friction. For example, a rubber- or neoprene-clad electrical cable may generate sufficient friction in a long run to break the cable.